Don't know if this is the sort of thing that you're looking for, but a while back, there was a Rolf Gauger column in Bowling This Month describing the spare-shooting system that he teaches. This spare system specifies starting points on the approach to be used for the key pins for right- and left-side spares, but is predicated on the bowler's ability to throw a straight ball at spares. I don't recall the exact details, but I could try to dig up the article if you like.
